Shiso Growing Calendar: Sowing, Pinching, Picking and Clearing

The year at a glance

Shiso is an annual that runs on temperature at the start of the season and on day length at the end of it. That combination makes the calendar predictable: you cannot rush the beginning, and you cannot postpone the ending. Everything useful you do happens in the middle.

Spring is mostly waiting

There is very little to gain from an early start with shiso, and quite a lot to lose. Seed in cold compost sits and rots, and seedlings that survive a cold spell sulk for weeks and are overtaken by anything sown a fortnight later into warm soil. If you cannot give a tray a steady 68F (20C), wait.

Where spring effort does pay is in preparing the bed. Shiso is not a hungry plant and it will grow soft and floppy in over-rich ground, but it does want soil that holds moisture, because the thin leaves wilt at the first sign of drought.

1. Sow indoors, four to six weeks before your last frost Surface-sow into fine compost with a clear cover for humidity. Expect ten to twenty-one days before anything shows, and do not throw the tray out at day ten.
2. Work compost into the bed A couple of inches of garden compost forked in improves moisture holding, which matters more for shiso than fertility does.
3. Wait for reliable warmth to plant out Shiso will not tolerate frost at all, and cold nights below 50F (10C) check it badly. Harden off over a week and plant when nights are settled.
4. Deal with last year's seedlings If you grew shiso here last year, a carpet of self-sown seedlings will appear as the soil warms. Decide now whether to keep a few in place or hoe the lot; leaving them undecided means a thicket.

Self-sown seedlings often outperform anything you raise in a tray, because they have never been transplanted. If they have come up somewhere convenient, thinning them to spacing is the least work for the best plant.

Early summer builds the plant

This is the part of the season with the highest return on effort, and the part most often skipped. Every pinch you make in the four to six weeks after planting out multiplies the number of shoots, and the plant you have by midsummer is entirely a product of that work. A shiso left unpinched through early summer cannot be made into a bush later.

Feed lightly if at all. Shiso in rich ground with plenty of nitrogen makes big, pale, floppy leaves with a weak scent, which is not what you grew it for.

1. First pinch at four pairs of leaves Take out the growing tip just above a joint. This is the pinch that decides everything after it. Eat the tip.
2. Pinch again every two to three weeks Work by leaf count: when a shoot has three or four pairs of leaves it is ready. Two shoots become four, four become eight.
3. Mulch before the heat arrives An inch of mulch put down while the soil is still moist saves an enormous amount of watering later and keeps flea beetles down.
4. Sow a second batch A sowing made now, four to six weeks behind the first, gives you young tender leaves at the point when the first plants are getting tough.

If you only do one thing all season, do the pinching. It has a larger effect on total harvest than watering, feeding or spacing.

High summer is about water

By midsummer a well-pinched plant is a bush a couple of feet across with a large leaf area and a correspondingly large appetite for water. In a container it can empty a ten inch pot in a single hot day. Shiso wilts dramatically and recovers if you catch it, but a plant that wilts repeatedly produces small, thick, bitter leaves for the rest of the season.

This is also peak harvest. Picking hard now does the plant good, keeps it in leaf growth, and delays flowering slightly.

Water the soil in the morning rather than splashing the foliage in the evening. Wet leaves overnight in warm weather is how leaf spot gets started.

Autumn, seed, and the plant that sows itself

As the nights lengthen, shiso flowers. Pinching out the spikes weekly holds it in leaf for another three or four weeks, but the direction is one way. When the leaves at the tips come small and narrow and the scent thins, harvest everything worth having and preserve it.

Then make a deliberate decision about seed, because shiso self-seeds prolifically. A single plant left to shed will produce hundreds of seedlings in the same spot next spring, and in mild regions it has escaped gardens and become a nuisance weed in field margins and waste ground. Choose one or two plants to save seed from, cut every other spike before the seed browns, and put them in the bin rather than the compost pile.

1. Cut spikes for seed into a paper bag Take whole spikes once the seed has gone hard and brown, bag them, and hang them somewhere dry for a fortnight before shaking the seed out.
2. Remove every other spike before it drops A spike cut while the seed is still green will not germinate. Left two weeks longer, it will shed as you carry it across the garden.
3. Clear the plants before frost Shiso is killed outright by the first frost and turns to slime. Pull the plants while they are still firm and easy to handle.
4. Hoe the seedling flush next spring Whatever seed you missed will germinate in a carpet as the soil warms. Hoe them off small; a shiso seedling with four true leaves already has a taproot that resists pulling.

Write down where each plant stood, when you made the first pinch, when the first flower spike showed, and roughly how long the picking lasted. Those four lines are all you need to plan next season's sowing dates and spacing.

Catching up if you are late

Missing a stage is not fatal, but the corrections differ depending on which one you missed. Work out where you are and take the shortest route back rather than trying to do the missed job now.

Because germination is slow, a rescue sowing needs at least ten weeks of warm weather ahead of it to be worth doing. Later than that, put the effort into pinching and watering the plants you already have.